A theoretical investigation of the influence of pentahydrated Mg2+ cation binding on the Hoogsteen G-G-C triplet motif is presented. Theoretical calculations have been performed by using the structural data of the G-G-C triplet found in the crystallographic structure of the B-DNA duplex d(GCGAATTCG) [J. Biol. Chem. 1999, 274, 23683-23686]. The investigation involves a variety of theoretical analysis, which includes analysis of geometrical, energetic, electron topology, and electrostatic potential properties. Calculations show the large stabilizing effect of the Mg2+ cation on the G-G-C triplet. Such a stabilization mainly arises from the balance of electrostatic interactions between the cation with the three bases in the triplet. However, the cation-induced polarization, which is largely concentrated in the Hoogsteen guanine, also plays a significant contribution to the stabilization of the triplet. The results can be valuable to gain further insight into the cation-induced enhancement of the stability of triple helical structures.